The invention relates to an observation device comprising a rangefinder, according to the preamble of Claim 1 and to a distance measuring method for an observation device, according to the preamble of Claim 11.
Such observation devices have diverse fields of application, for example in hunting, for landmark navigation on land or at sea, for aiming at objects, for acquiring and documenting geographic surroundings, as information device for hikers, etc. In addition to such civilian fields of application, such devices are also used in the military sector for navigation, observation, etc. Here, it is important for the device to be robust, convenient, operable in a quick and simple manner, and as compact and as light as possible.
The observation devices within the scope of the present invention are robust devices designed for use in the field. These devices are not highly precise and correspondingly sensitive geodetic surveying devices such as tachymeters or theodolites with measurement resolutions in the millimeter range or with higher measurement resolutions, which are used e.g. in the construction sector. The devices described here usually have measurement resolutions of the order of meters or, at best, decimeters, but have measurement ranges of several kilometers, for example of up to five or thirty kilometers or more. The observation devices are primarily provided for hand-held use by persons, i.e., for example, as field glasses or binoculars, monocular telescopes, spotting scopes, portable weapon systems, etc., but can by all means be assembled on a tripod or the like if necessary.
The observation devices treated here can, in particular, comprise an optical transmitted light channel—that is to say, be conventional optical devices in terms of the basic function thereof, in which optical radiation is directed directly from the observed target object into the eye of the observer. However, in other embodiments, these can also be observation devices in which an observation image is recorded using a camera, the image is converted into electrical signals and the electrical signals are reproduced for the observer on a screen display. Here, especially in the case of the devices with a screen, the observation through an eyepiece, through which the recorded observation image can be observed, can be brought about in the conventional manner. Here, the observation path can by all means comprise optical elements for beam shaping, beam deflection, mirroring information in and out, amplifying residual light, etc. Specifically, this can relate to hand-held observation devices or distance-measuring observation devices which are generically embodied for use as a hand-held device, for example by appropriate handles, shaping, etc.
Here, the optical targeting using the observation device also determines the direction of the distance measurement. Here, the point to be measured is targeted by means of an optical apparatus of the transmitted light channel, for example by means of crosshairs in the observation apparatus of the device. In the case of electro-optical distance meters or rangefinders (EDM), an optical signal, for example as optical radiation in the form of laser light pulses, is emitted by the device in the direction of the target object, the distance of which is intended to be determined. If visible light is used in the process, the point on the target object targeted for measuring purposes can be identified visually in the case of appropriate light conditions. However, non-visible wavelengths, e.g. in the infrared range, are often used and the point on the target object targeted for measuring purposes is determined for the user purely by targeting with the observation channel of the device.
The surface of the target object casts back at least a portion of the emitted optical signal, usually in the form of a diffuse reflection. In the device, the cast-back optical radiation is converted into an electrical reception signal by a photosensitive detector element. The distance between the device and the target object can be determined with knowledge of the propagation speed of the optical signal and on the basis of the determined travel time between emission and reception of the signal (i.e. the travel time which light requires for covering the distance from the device to the target object and back again). Here, there usually are one or more optical components for beam shaping, deflection, filtering, etc.—such as lens elements, wavelength filters, mirrors, etc.—in the optical transmission or reception path. Here, transmission and reception can be brought about coaxially using a single optical unit or separately using two separated optical units (e.g. arranged next one another). Here, the distance meter or the rangefinder is integrated in the observation device.
In order to compensate influences which could falsify the measurement results (e.g. temperature influences, component tolerances, drifts of electronic components, etc.), part of the emitted optical signal can be guided as a reference signal over a reference path of a known length from the light source to a light-sensitive reception element. Here, the reference path can be fixedly installed in the device or, for example, be embodied as an optical deflection element that can be pivoted in or plugged on. The reception signal resulting from this reference signal can be received by the photosensitive element used for the target distance measurement or by a further photosensitive element provided especially for the reference signal. The resultant electrical reference signal can be used for referencing and/or calibrating the determined distance measurement values.
In order to obtain a correspondingly high accuracy of the distance measurement, the demands placed on the temporal resolution capabilities of the electro-optical distance meter (EDM) are relatively high due to the high propagation speed of optical radiation. By way of example, for a distance resolution of 1 m, a time resolution with an accuracy of approximately 6.6 nanoseconds is required.
The measurement requires sufficiently strong signal intensities, which can be detected by the receiver, of the returning reception signal. However, the signal power that can be emitted of the transmission signal of the optoelectronic EDM considered here is restricted by physical and regulatory limits. Therefore, work is often undertaken with pulsed operation. Thus, the intensity amplitude of the emitted optical signal is modulated in a pulse-like manner. Temporally short pulses with a high peak power are emitted, followed by pauses during which no light is emitted. Hence, the cast-back component of the pulses has a sufficiently high intensity to allow these to be evaluated from background disturbances and noise, in particular even in the presence of background light (sunlight, artificial illumination, etc.).
In the case of observation devices with rangefinders, ranges from several meters up to many kilometers, for example from 5 m to 20 km or 30 km, are required in this case, and this is required with a measurement accuracy of several meters or even higher measurement accuracy, for example of ±5 m or ±1 m or less. Since, in general, the measurement target does not have special reflective target markers for the measurement (as is conventional in measurement rods, measurement prisms etc. used in surveying), the applied optical distance measurement signal must be embodied and set in the device design in such a way that a distance measurement is possible over the whole specified measurement range (or the range must be specified on the basis of the possibilities of the used signal). Since only a small portion of the emitted radiation returns to the receiver in the case of natural or non-cooperative targets, the signal information from a plurality of pulses is used cumulatively (in particular in-phase) for the evaluation. In the process, the signal-to-noise ratio (SNR) is improved in order thereby also to enable measurements in disadvantageous conditions. By using a plurality of measurement light pulses on the same target point, disturbance signals are removed by averaging and the used signal is amplified, corresponding to a theoretical SNR improvement of approximately the square root of the number of accumulated pulses.
In order to determine the travel time of the signal, the so-called time of flight (TOF) process is known on the one hand, which determines the time between the emission and reception of a light pulse, with the time measurement being performed on the basis of the flank, the peak value or another characteristic of the pulse shape. Here, the pulse shape should be understood to mean a temporal light intensity profile of the reception signal, specifically of the received light pulse—acquired by the photosensitive element. The transmission time can be determined on the basis of an electrical pulse for triggering the emission, on the basis of the actuation signal applied to the transmitter or on the basis of an aforementioned reference signal.
Alternatively, the so-called phase measurement principle is also known, which determines the signal travel time by comparing the phase angle of the amplitude modulation of the emitted and received signals. However, in this case, the measurement result at a transmission frequency has ambiguities in units of the transmission frequency period duration, and so further measures are required for resolving these ambiguities. By way of example, WO 2006/063740 discloses measuring using a plurality of signal frequencies resulting in different uniqueness ranges, as a result of which incorrect solutions can be excluded. WO 2007/022927 also treats uniquenesses in the phase measurement.
In a typical use scenario, the user targets a desired target using the observation device and then triggers the distance measuring process, for example by actuating a trigger button or the like. Thereupon, the measurement result, or further in-depth information derived therefrom, such as coordinates, etc., is displayed to said user, preferably directly by means of the observation channel of the observation device.
The observation device can be equipped with means for determining geographic coordinates, such as a GPS, a constellation identifier, a direction measuring unit, a compass unit, tilt sensors or accelerometers, a night vision function, etc. Using an electronic display for providing information, it is possible, for example, to provide to the user in the transmitted light channel an image from a camera, location information, for example in the form of a map, measured distances or directions, stored information in respect of a sighted target object, temperature and weather information using the electronic display. Depending on field of application and demands on the respective measurement situation, the observation device may, in a modified embodiment, be equipped with e.g. a night vision module, etc. In this context, EP 1 744 196 proposes, in an exemplary manner, several different embodiments for a generic observation device, for example for target marking, for military applications or for hunting purposes.
In the case of a hand-held observation, instabilities and movements of the device as a result of being held in the hand are to be expected in this case, especially in the form of oscillations or oscillation-like movements as a result of trembling, swaying or twitching of the user. This has a clear visible effect, particularly in the case of faraway targets and high magnifications. In the case of distances of the order of kilometers, small changes in angle of the targeting direction already cause lateral displacements of the observed target in the image plane corresponding to several meters. Therefore, continuous, exact targeting of a comparatively small and faraway target is often difficult for the user using a hand-held observation device and requires great concentration and body control. Similar variations in the spatial position of the device can also occur when using the device on an unstable base, such as a land vehicle, aircraft or water vehicle, or when the ground shakes.
As a result of the movements of the observation device, the distance measurement with inclusion of a plurality of measurement light pulses is no longer directed on the same target point, but on a multiplicity of different points which, at least in part, may have varying distances. Therefore, in such a case, the application of a combination of information from a plurality of pulses for determining the distance only brings about a slight improvement in the SNR compared to what is promised from the superposition of information from a plurality of pulses in the theory. This deteriorated SNR can lead to relatively large measurement uncertainties, relatively large measurement errors or even to a failure of the measurement. Further lengthening of the measurement duration for emitting further pulses for improving the SNR, firstly, is undesirable and, secondly, only has limited efficiency due to the further target point deviations occurring thereby.
In the prior art, active dampening or prevention of movements by using a rod or tripod for supporting the device are applied for avoiding trembling movements as a result of being held by hand. However, such additional outlay for setting up the device is undesirable, in particular in view of the primary design as hand-held device.
Mechanical movements of optical elements in the device interior are another known solution for actively stabilizing the direction of observation devices; however, it is usually complicated in terms of the realization thereof, reduces the robustness of the device and makes the device larger and heavier. Moreover, such active stabilizations need to be supplied with power.
The field of digital photo and video equipment has also disclosed digital stabilization of an observation image from a hand-held device using a purely screen-based observation. Although such a digital image stabilization can optionally also be present in the observation devices of the present invention, it is then functionally separated from the approach according to the invention which makes do without image processing or information obtained therefrom. Thus, the stabilization of the distance measurement according to the invention is not dependent on information generated by digital image processing.
Electronic components required for digital stabilization use up e.g. additional power and reduce the robustness in rough usage surroundings, for example by the restricted operational temperature ranges of LC displays and CCD cameras. Restricted optical dynamic ranges of these elements and the high computational complexity for digital processing of image information are further undesirable side effects of such solutions. By contrast, a special embodiment of an observation device according to the invention, with an optical transmitted light channel can continue to meet the basic functionality thereof—a magnified observation of distant objects—even without electrical energy, for example in the case of a defect or empty battery, e.g. in the case of long-term use, and is not only degraded to useless high-tech ballast. Moreover, in such embodiments, the expected battery life is significantly increased.
Document FR 2 921 149 describes a digital device from telemetry, comprising a camera recording unit and an electronic display for displaying the recorded image. In order to depict for the user a clear observation image despite the movements as a result of being held by hand, the recorded camera image is numerically stabilized before being displayed. Since information about the actual current target direction is lost during this stabilization, the latter is additionally also displayed in the stabilized image on the display. Here, a distance to a target object is measured in two steps. Firstly, the desired target is targeted using crosshairs in the stabilized image on the display, which is confirmed by triggering a measurement query. However, the distance measurement is not triggered directly in this case with the actuation of the triggering. First of all, the user must attempt in a second step to bring the current target direction of the distance measurement, depicted in the display, into sufficient correspondence with the crosshairs in the stabilized image by moving the alignment of the observation device. If the device determines the correspondence, it now emits a single light pulse for measuring the distance at this later time, on the basis of which the distance to the target in the crosshairs of the stabilized image is determined. Here, the above-described system merely solves the problem of an unknown target direction of the image-based, image stabilized distance measuring observation device, which problem was newly introduced with the image stabilization. In addition to the previously mentioned disadvantages of fully electronic observations and digital image processing of image stabilizations, etc., this system comprises. Moreover, the system from FR 2 921 149 is based on an individual pulse measurement, in which signal information from a plurality of pulses are not combined to form a measured value, and so, instead, the target can only either be hit or missed.
FR 2 965 935 likewise discloses an observation device with indirect, camera-based observation with a digital stabilized observation image, in which there is an accumulation of a plurality of laser pulses for determining the distance. Here, a sequence of light pulses is emitted when the measurement is triggered. The returned echoes are linked to measurement data from a gyroscope measurement of the current trembling movements—as directional deviation in relation to the target direction of the stabilized display image. The echoes are assigned to corresponding zones in the target region on the basis of this angular deviation. The measured values from the zone in which the best SNR value is obtained are used for determining the distance, which is presented to the user.